This blog post examines the air pollution problem from diesel vehicles and the limitations and potential of aftertreatment technology to address it.
Seoul is always an exciting place. When you take the KTX from Busan Station, ride for two and a half hours, and get off at Seoul Station, you’re often startled. Especially, the first thing that catches your eye when you arrive at Seoul Station is the hazy, murky sky. Just three hours earlier, the sky was clear and transparent in Busan, but in Seoul, it’s often turned a grayish hue. This air pollution stems from Seoul’s much larger population and vehicle count compared to Busan. While Seoulites, who encounter this gray sky daily, have grown somewhat accustomed to it, this is not solely Seoul’s problem. Deteriorating air quality and smog have long been established as serious environmental issues facing major cities worldwide.
Indeed, the problem of air pollution caused by automobile exhaust has been a subject of social debate since the early 20th century. For example, an automobile museum in Alaska exhibits protective gear worn by ladies in the early 1900s to prevent their clothes or skin from being soiled by exhaust fumes, demonstrating that air pollution was recognized as an issue from the very beginning of the automobile era. The problem of air pollution caused by automobiles has grown increasingly severe over time, leading many countries, including the United States and Europe, to strengthen regulations on vehicle emissions in modern times.
Particularly, concerns about air pollution are intensifying due to the rising demand for diesel engines, which emit more pollutants than gasoline engines. While diesel engines boast high fuel efficiency, they come with the trade-off of emitting more pollutants. Consequently, various aftertreatment devices and technologies to reduce pollutants from diesel engines are being developed, and their importance is increasingly recognized.
The difference between diesel and gasoline engines begins with the physical properties of the fuel. Relatively lighter components from petroleum are classified as gasoline, while heavier components become diesel. The weight difference between these fuels leads to a difference in their boiling points (the temperature at which they vaporize), which in turn dictates how they are injected into the engine. Gasoline is pre-mixed with air before entering the engine, where combustion occurs in a uniformly mixed state. This allows gasoline to achieve relatively complete combustion. Conversely, diesel fuel, with its higher boiling point, enters the engine separate from air. Fuel is injected and combusted after air is forced in under high pressure. This difference causes diesel engines to emit fuel particles that have not fully reacted with oxygen. These particles are the harmful particulate matter (PM) that is one of the primary air pollutants emitted by diesel vehicles.
Another pollutant generated by diesel engines is nitrogen oxides (NOx). Because diesel engines operate at high temperatures and pressures, stable nitrogen molecules in the atmosphere combine with oxygen to form nitrogen oxides. Nitrogen oxides trigger chemical reactions in the atmosphere, leading to the formation of ozone and fine dust. This can cause respiratory and cardiovascular diseases over the long term. Meanwhile, substances like nitrogen oxides contained in vehicle exhaust are generally invisible and thus easily overlooked. However, when pollution accumulates, it can cause smog in the atmosphere and lead to long-term environmental problems.
Various aftertreatment devices have been developed to mitigate the air pollution issues from diesel vehicles. A representative device is the Diesel Particulate Filter (DPF), which traps particulate matter in exhaust gases through a filter, reducing the millions of particles emitted by the engine to just thousands. The DPF works by trapping particulate matter inside its closed-end filter as exhaust enters, allowing only clean exhaust to exit. To maintain this filter’s function, a ‘regeneration’ process must be performed periodically. This involves raising the internal temperature to burn off the accumulated particulate matter. While such devices can effectively reduce particulate matter, neglecting filter maintenance can cause the filter to clog, potentially leading to increased pollutant emissions. Therefore, thorough management is essential.
Another aftertreatment device is the Exhaust Gas Recirculation (EGR) system. EGR recirculates a portion of the exhaust gas back into the engine combustion chamber to lower combustion temperatures, thereby suppressing nitrogen oxide (NOx) formation. While EGR reduces NOx emissions, it has the drawback of potentially affecting engine efficiency. Additionally, the Selective Catalytic Reduction (SCR) system has been developed to chemically neutralize nitrogen oxides. SCR enhances exhaust purification efficiency by injecting a material containing a reducing agent, which decomposes nitrogen oxides in the exhaust into water and nitrogen. This technology is particularly widely used by European automakers like Mercedes-Benz and BMW.
However, the presence of these aftertreatment devices does not solve all problems. Automotive aftertreatment systems only function properly above a certain temperature. They may fail to operate effectively during short trips or repeated low-speed driving in urban areas. For instance, most pollutants are emitted intensively within about 10 minutes after starting the engine, a period when aftertreatment systems often cannot function effectively. Furthermore, certain aftertreatment devices have issues such as increasing fuel consumption or potentially causing the generation of other pollutants, meaning many challenges remain to be addressed.
As concerns about air pollution grow, regulations on automotive exhaust emissions are tightening globally, driving active technological development to reduce pollutants. Improving the technology to ensure diesel engine aftertreatment systems maintain consistent efficiency and stability under various driving conditions will remain a critical challenge. To pass on a better environment to the next generation, the automotive industry demands the development of enhanced emission reduction devices and innovative technologies. This will progressively contribute to providing cleaner air in our daily lives.
